# I did the math: single 6374 beats dual 6354

Sorry for the clickbait title, but I did the math, and it suggests that single 6374 basically just as powerful as dual 6354s. Obviously this seems wrong and I’m still pretty new here and learning a ton, but I can’t find the mistake and I’d REALLY like to get some feedback. Let me know what you think.

Comparing a few setups:

1. single 6374, ~200 Kv (lets assume SK3)
2. dual 5045, ~200 Kv (only suitable Kv motor I found were Racestar’s, everything else seems to be Kv 500+)
3. dual 90mm hub, ~85 Kv (essential dual 6364 motors, seem to be popular hub motors with brands like Meepo, WowGo, etc)
4. dual 6354, ~200 Kv (several suppliers have these)

At any given time: Power_in = Power_out - Power_lost

Power_in is basically what your battery and ESC can put out, so something like 36V * 50A ish

A “powerful” motor setup is the one that converts as much of the electrical power from the battery into mechanical power at the wheels as possible, i.e. we need to maximize Power_out

That means we need to minimize Power_lost, which comes from air-resistance, rolling resistance, belt-drag, and copper losses (resistance * current^2). There’s nothing we can do about air-resistance and rolling resistance, so ignore those. Lets also ignore belt-drag for now since that 3 of the 4 setups use belts, and looking at some manufacturer numbers belt drives should be on the order of 95% efficient anyway. The only thing left to try to minimize is the copper losses. So lets estimate the copper losses for each setup, starting with the motors’ internal resistances.

1. SK3 6374 has a internal resistance of 0.016 ohm according to HK

2. Estimating the Racestar 5045 200 Kv internal resistance is tricky since its not quoted anywhere. But we can guess based on the SK3 5045, 450 Kv motor, which has an internal resistance of 0.025 ohm according to HK. Kv goes down when wire turns go up, and if you increase wire turns you have to use thinner wire, so basically a motor’s internal resistance scales as the square of the inverse of Kv. So roughly speaking, a 200 Kv Sk3 should have an internal resistance of 0.025 ohm * (450 Kv / 200 Kv)^2 = 0.127 ohm, roughly 8x that of the 6374

3. Again we have to estimate the internal resistance of these hub motors, but basing it off of SK3 6364 190 Kv motors gives us an estimate of 0.028 ohm * (190 Kv / 85 Kv)^2 = 0.14 ohm, roughly 9x that of the 6374

4. Estimated internal resistance based on SK3 6354 260Kv motors gives us 0.023 ohm * (260 Kv / 200 Kv)^2 = 0.039 ohm, or roughly 2.5x

5. the single 6374 will suck all of the current from the battery (I_6374), so it’s copper losses are I_6374^2 * R_6374, where R is it’s internal resistance

6. each motor in a dual 5045 setup will only consume half of the current, and R_5045 = 8 * R_6374, so the total copper losses across both motors is basically 2 * I_5045^2 * R_5045, 2 * (0.5 * I_6374)^2 * 8 * R_6374 = 4 * I_6374^2 * R_6374. You’re losing four times as much power to copper-losses compared to a single 6374

7. 2 * I_hub^2 * R_hub = 4.5 * I_6374^2 * R_6374, 4.5x the power losses

8. 2 * I_6354^2 * R_6354 = 1.25 * I_6374^2 * R_6374, 1.25x the power losses

So basically, single 6374 is within error of equivalent to dual 6354 in power, and 4x more powerful than dual 5045 or dual 90mm hubs.

Durability wise, dual 6354 is better than single 6374 since the heat is easier to dissipate over two separate motors. And traction is way easier to manage on dual 6354s. But consider that adding a second 63XX motor costs \$20 truck mount + \$40 belts and pulleys + \$80 motor + \$100 VESC = \$240, not to mention weight and complexity, it seems to me that budget or light-weight builds should seriously consider the single 6374 as an extremely compelling alternative.

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Read up on belt slippage - case closed.

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Sure looks complicated…is Esk8 that complicated?

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Just be dumb like us and use this thing http://calc.esk8.it/ and start building all those decimals are giving me a headache

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Maxid, that’s a great point with conventional drivetrains. If we bump up to 8mm tooth pitch and 15mm belt width though, it wouldn’t cost much in weight or \$\$, and I think it would solve that problem.

Michaelinvegas, haha no it doesnt need to be at all! But there are plenty of people trying to build light weight or inexpensive boards, who I hope might find this kind of stuff useful.

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Here’s a paper someone did on eksates for their Master’s Thesis from Massey University in New Zealand.

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holy fkn shit, did they get their thesis approved?

210 pages, wow

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What about in terms of traction? IMO two smaller motors will always beat one bigger motor

Can’t be published if it is not approved.

It’s 108 pages - everything past the bibliography is basically unnecessary. Can’t believe people get a masters degree for this type of work in 2016 - “An electric powered skateboard was designed and built for testing and development of an innovative hub motor propulsion system and motor controller.” @jacobbloy should have gotten a PhD then.

PS: Yes I am salty and yes I am still reading this thing.

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If you can TL;DR it for us that would be awesome

well he also designed his own motor controller to do FOC tests so the work feels adequate - but he probably could have just used a VESC and do the tests this way. Without the time frame it took to finish the thesis it is hard to judge. The code comments say 2013 so I am not sure if it took him 3 years to finish (which seems really long for a Masters thesis)

Its not rocket science really it isn’t. Driving both rear wheels is a lot better than driving one. Stop doing math and build it and ride it and see.

I’m gonna agree with the above, you’re overthinking the motor efficiency. On the relevant scale of battery capacity in esk8, one motor vs dual motor efficiency is not a “make or break” decision point. I mean, in terms of watt hr/mile (Km, sorry), the range of “x” battery running single vs dual motor is not a deciding factor.

What is a deciding factor is the cost of another motor and esc, belt, and gears. The feels of a single vs dual are similar, in that they’re both an electric-fucking-miracle-vehicle, but in the scope of “EFMV’S”, Dual motor is worth the extra money for most of us here at miracle vehicle builders dot forum.

Back on topic, if you’re NOT limited by money, dual drive is going to be a better experience for the rider, and the cost/benefit is only really measurable in dollars or pesos. If you aren’t stranded in the desert, don’t waste time comparing efficiency, it’s about what you want to ride!!!

Tldr: practically speaking, you’re gonna use similar amounts of energy either way. Mechanically speaking, dual drive wears out half as fast (and there’s other pros&cons) has better brakes and makes your friends precisely 2.25 times more jealous

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Sometimes I feel like that esk8s are still comparatively cheap. I mean even a full blown 4WD Trampa ATB setup for like 3000\$ is still “just” the price people pay for their in-car-radio or non-standard car paint. It’s crazy…

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Agreed! If what a human being wants is a means of transportation, it’s a “sky’s the limit” sort of thing as far as what you can spend. Or you can walk for free. @Jmding you have many things to consider in the long run, and with enough research and cost benefit analysis, you may find exactly what you hope for. My opinion is that overkill is relatively similar in cost to “mediocre” and building for performance is the more rewarding (albeit more expensive) approach. But again, I’m a shameless hardware junkie

I think one of the drawbacks is braking power and acceleration when using only one motor.

If u design a transmission / use differential to drive 2 wheels at once with one motor, then maybe it is a different story but u would probably lose some effienciency in this drive train

Grip… on my single dark matter powered board I have all the power I need for any hill, until it’s rough or damp then I lose all that torque into wheel spin. Same goes with knobs on for braking.

It wasn’t that big a deal for the first 100 miles but now I hit my max speed every ride for most of the ride and at 30mph the braking with one wheel drive is too sketchy!

Dual 6374 is the way forward! (Why limit yourself to dual 6354!)

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Here’s the comparison between (2) 200kv motors with 0.039ohm vs (1) 200kv motor with 0.016ohm…

During acceleration at full throttle, the single motor with lower resistance has higher electrical to mechanical conversion efficiency (green line, top left chart), but half as many pounds of thrust as the dual motor with higher resistance (yellow and green lines, bottom left chart). Ohmic heating per motor is roughly 2.5 times greater with the dual motor with 0.039ohm resistance.

Both systems are assumed to have 50v battery, 200kv, 90mm tire, 2.55:1 gear ratio, 100% throttle, 95% max duty cycle, 60a motor amp limit and 30a battery amp limit.

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Professor, why is that? Single and dual are both consuming the same total current at the same Kt, so shouldn’t the torque be the same?